mm: Cast the type of unmap_start to u64
Subsystems (2)
PATCH 1/1 mm: Cast the type of unmap_start to u64
In 64bit system. sb->s_maxbytes of shmem filesystem is MAX_LFS_FILESIZE, which equal LLONG_MAX.
If offset > LLONG_MAX - PAGE_SIZE, offset + len < LLONG_MAX in shmem_fallocate, which will pass the checking in vfs_fallocate.
/* Check for wrap through zero too */ if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0)) return -EFBIG;
loff_t unmap_start = round_up(offset, PAGE_SIZE) in shmem_fallocate causes a overflow.
Syzkaller reports a overflow problem in mm/shmem:
UBSAN: Undefined behaviour in mm/shmem.c:2014:10 signed integer overflow: '9223372036854775807 + 1' cannot be represented in type 'long long int' CPU: 0 PID:17076 Comm: syz-executor0 Not tainted 4.1.46+ #1 Hardware name: linux, dummy-virt (DT) Call trace: [<ffff800000092150>] dump_backtrace+0x0/0x2c8 arch/arm64/kernel/traps.c:100 [<ffff800000092438>] show_stack+0x20/0x30 arch/arm64/kernel/traps.c:238 [<ffff800000f9b134>] __dump_stack lib/dump_stack.c:15 [inline] [<ffff800000f9b134>] ubsan_epilogue+0x18/0x70 lib/ubsan.c:164 [<ffff800000f9b468>] handle_overflow+0x158/0x1b0 lib/ubsan.c:195 [<ffff800000341280>] shmem_fallocate+0x6d0/0x820 mm/shmem.c:2104 [<ffff8000003ee008>] vfs_fallocate+0x238/0x428 fs/open.c:312 [<ffff8000003ef72c>] SYSC_fallocate fs/open.c:335 [inline] [<ffff8000003ef72c>] SyS_fallocate+0x54/0xc8 fs/open.c:239
The highest bit of unmap_start will be appended with sign bit 1 (overflow) when calculate shmem_falloc.start:
shmem_falloc.start = unmap_start >> PAGE_SHIFT.
Fix it by casting the type of unmap_start to u64, when right shifted.
This bug is found in LTS Linux 4.1. It also seems to exist in mainline.
--- mm/shmem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
mm/shmem.c +1 -1
@@ -2717,7 +2717,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset, } shmem_falloc.waitq = &shmem_falloc_waitq; - shmem_falloc.start = unmap_start >>PAGE_SHIFT ; + shmem_falloc.start = (u64 )unmap_start >>PAGE_SHIFT ; shmem_falloc.next = (unmap_end +1 ) >>PAGE_SHIFT ;spin_lock (&inode->i_lock); inode->i_private = &shmem_falloc;
Signature · 1 line
2.7.4
Quoted 8 lines · Chen Jun
In 64bit system.
sb->s_maxbytesof shmem filesystem isMAX_LFS_FILESIZE, which equalLLONG_MAX.
If offset >LLONG_MAX-PAGE_SIZE, offset + len <LLONG_MAXinshmem_fallocate, which will pass the checking invfs_fallocate./* Check for wrap through zero too */ if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0)) return -EFBIG;
Quoted 5 lines
loff_tunmap_start=round_up(offset, PAGE_SIZE)inshmem_fallocatecauses a overflow.Syzkaller reports a overflow problem in mm/shmem:
UBSAN: Undefined behaviour in mm/shmem.c:2014:10
What is the syzkaller reproducer if any?
Quoted 41 lines
signed integer overflow:
'9223372036854775807 + 1' cannot be represented in type 'long long int'CPU: 0 PID:17076 Comm: syz-executor0 Not tainted 4.1.46+ #1 Hardware name: linux, dummy-virt (DT) Call trace: [<ffff800000092150>] dump_backtrace+0x0/0x2c8 arch/arm64/kernel/traps.c:100 [<ffff800000092438>] show_stack+0x20/0x30 arch/arm64/kernel/traps.c:238 [<ffff800000f9b134>] __dump_stack lib/dump_stack.c:15 [inline] [<ffff800000f9b134>] ubsan_epilogue+0x18/0x70 lib/ubsan.c:164 [<ffff800000f9b468>] handle_overflow+0x158/0x1b0 lib/ubsan.c:195 [<ffff800000341280>] shmem_fallocate+0x6d0/0x820 mm/shmem.c:2104 [<ffff8000003ee008>] vfs_fallocate+0x238/0x428 fs/open.c:312 [<ffff8000003ef72c>] SYSC_fallocate fs/open.c:335 [inline] [<ffff8000003ef72c>] SyS_fallocate+0x54/0xc8 fs/open.c:239The highest bit of
unmap_startwill be appended with sign bit 1 (overflow) when calculate shmem_falloc.start:
shmem_falloc.start =unmap_start>>PAGE_SHIFT.Fix it by casting the type of
unmap_startto u64, when right shifted.This bug is found in LTS Linux 4.1. It also seems to exist in mainline.
Signed-off-by Chen Jun <[email protected]>--- mm/shmem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)mm/shmem.c +1
-1@@ -2717,7 +2717,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset, } shmem_falloc.waitq = &shmem_falloc_waitq; - shmem_falloc.start = unmap_start >>PAGE_SHIFT ; + shmem_falloc.start = (u64 )unmap_start >>PAGE_SHIFT ; shmem_falloc.next = (unmap_end +1 ) >>PAGE_SHIFT ;spin_lock (&inode->i_lock); inode->i_private = &shmem_falloc;